1. Field of the Invention
This invention generally relates to a signal transmission structure, and more particularly to a signal transmission structure for improving the characteristic impedance mismatch of the signal transmission structure.
2. Description of Related Art
The signal line in a printed circuit board with a large scale or a packaging substrate for connecting two devices or two terminals has to maintain a uniform line width in order to keep a constant characteristic impedance when an electronic signal is transmitted in the signal line. A good impedance matching design between the two terminals are required to reduce the reflection due to the impedance mismatch especially when the signal is transmitted in a high speed and a high frequency environment i.e., to reduce the insertion loss and increase the reduction of the return loss when transmitting the signals so that the quality of the signal transmission will not be affected.
FIGS. 1A and 1B show a top view and a side view of a conventional signal line passing through a non-reference area. The signal transmission structure 110 includes at least a reference plane 120 and a signal line 130. The reference plane 120, for example, can be a power plane or a ground plane. The signal line 130 has a uniform line width. It is important to note that in a conventional circuit design, the reference plane can form a plurality of through holes due to the hole drilling or cutting between the planes, or a non-reference area 122 (such as a non-reference area opening) can be formed to prevent adjacent signal lines from short-circuit. Hence, a high impedance occurs at the non-reference area 122 when the signal is transmitted in the signal line 130 so that the impedance mismatch causes the increase of the insertion loss. Therefore, the signal cannot be transmitted without a loss from one terminal of the non-reference area 122 to the other terminal of the non-reference area 122.
FIG. 2A shows the frequency response S21 when the conventional signal line passes through a reference plane (solid line R1) and a non-reference area (solid line T1). Referring to FIGS. 1A and 2A, the return loss is more decreased when the working frequency is high i.e., the distortion is more serious. This is due to that the conventional signal line passing through a non-reference area 122. FIG. 2B shows the frequency response S11 when the conventional signal line passes through a reference plane (solid line C2) and a non-reference area (solid line T2). Referring to FIGS. 1B and 2B, the insertion loss is also increased when the working frequency is high i.e., the distortion is more serious. Again it is due to the conventional signal line passing through a non-reference area 122.
FIG. 2C shows the relationship between the frequency and the characteristic impedance when the conventional signal line passes through a reference plane (solid line R3) and a non-reference area (solid line T3). Referring to FIGS. 1B and 2B, the characteristic impedance is high when the working frequency is high. Once again this is due to the conventional signal line passing through a non-reference area 122. As a result, the impedance mismatch occurs.
In light of the above, the frequency increases, the return loss decreases and the corresponding characteristic impedance increases when the signal line passes through the non-reference area. Hence, the difference in the characteristic impedance from the original design is increased so that the impedance mismatch is much more serious. Therefore and for the foregoing reasons, there is a desperate need for a method or structure that is able to improve the characteristic impedance mismatch when the signal line passes through a non-reference area.